The emergence of new small-scale vapor compression refrigeration systems has created an opportunity to create portable or wearable refrigeration systems, and often times these new applications require operating the compressor in non-vertical orientations and/or under various accelerations and gravity levels. One such application is in the thermal management of various electronic components, such as microprocessors, electronics, telecommunications, and guidance equipment on board terrestrial or aerospace vehicles. Another application is the man-portable cooling system for thermal protection of aviators, soldiers, emergency response teams, and hazardous materials handlers. Yet other applications include compressor based systems, such as cooling systems, for use in zero to low gravity environments in space. Some of these systems place special requirements on compressors not previously encountered in stationary refrigeration systems in that the oil lubrication that previously relied on gravity based oil sump is no longer usable due to lack of gravity or to orientation of the compressor being not in line with the gravity all of the time. In applications for which larger system sizes can be tolerated, such as fluid displacement devices, e.g., with over 20 cc of displacement per cycle, an external oil management system consisting of separation, collection and circulation functions can be used to enable operation of the compressor in any orientation. However, for most of the above-mentioned new applications, it is preferred or required that the compressor and refrigeration system be ultra-lightweight, highly compact (thus making an external oil management system undesirable) and perform reliably and efficiently in arbitrary orientations and under varying levels of gravity or accelerations.
Several types of compressors are currently available for use in refrigeration systems. For home refrigerators and air conditioners, rolling piston compressors, also referred to as fixed (or stationary) vane rotary compressors, are commonly used. In such a compressor, the vane does not rotate along with the rotor, but instead reciprocates in a slot enclosed by the stationary part of the compressor. The cylindrical part of the compressor that is mounted on the eccentric shaft is named a rolling piston because it appears to roll on the cylindrical surface of the cylinder wall. During the suction portion of a rolling piston compressor cycle, refrigerant gas is drawn through an inlet port into the rotary chamber, increasing the gas volume. Compression process takes place on the opposite side of the piston and vane, where the volume of gas decreases due to the eccentric motion of the roller. Discharge flow is controlled via a discharge valve.
While the small size (for a given capacity) of rolling piston compressors is advantageous, the leakage of refrigerant along the surfaces of the cylinder wall has to be maintained low enough to ensure high performance. Lubricating oil that is used in the compressor performs two functions essential to the proper functioning of the compressor's pump parts. The first function pertains to the lubrication of the moving parts themselves, in order to reduce frictional losses and prolong the life of the machine parts. The second function pertains to the sealing of all clearances between the moving parts and stationary parts, in order to minimize direct gas leakage that would adversely affect the capacity and efficiency of the compressor. Although the lubricating oil performs the above two essential functions inside of the compressor, once the oil leaves (along with the refrigerant) the compressor, the presence of oil in the refrigerant is not desirable as it is detrimental to the refrigeration system in many ways. For example, the oil coats the surfaces of the heat exchangers and thereby increasing the thermal resistance and lowering the heat exchanger effectiveness; it increases the pressure drop inside heat exchangers and thus draining energy and lowering the capacity and efficiency; it decreases the heat exchanger capacity; and etc. In short, the lubricating oil may be necessary and desirable inside of the compressor but utterly unnecessary and highly undesirable outside of the compressor in a refrigeration system. Further, if the oil leaving the compressor through the discharge tube can be minimized, the total amount of oil in the compressor and in the entire system can be reduced without detrimental effect. Lower volume of oil can result in the reduction of the volume of the compressor itself. Therefore, it is highly desirable to minimize the amount of oil entrained/mixed in the refrigerant going out of the compressor and traveling though the refrigeration system.
In the case of a household refrigerator or any other stationary or on-board refrigeration systems using a compressor, one is cautioned against storing, transporting or operating a refrigerator in any direction other than vertical or close to vertical within a narrow range. This near vertical orientation is necessary, otherwise the lubrication oil will either leak out of the oil sump located at the bottom of the compressor or not be sucked in to lubricate the machinery, and if the compressor is operated without the lubricating oil in proper places, the compressor will most likely become damaged prematurely or the motor can burn out due to increased friction. In the case of a portable cooling system worn by a person or transported in vehicles, airplanes, or in space, there is no easy way to ensure that the orientation of the compressor will be maintained close to vertical at all times unless the entire system is gimbaled, which in most cases is impractical. Consequently, in these above cases, the oil may not be in the oil sump or the oil in the sump will not be available for lubrication on start-up or during operation. Further, when the gravitation field is weak, such as in space, earth orbits or in systems undergoing accelerations that will alter the effective gravitational fields, the traditional sump arrangement at the bottom of the compressor casing will not function properly to provide necessary lubrication for the moving/rotating parts of the compressor. As described above, there are special lubrication and oil management requirements for compressors and other machinery used in portable applications in general and under rapidly changing accelerations or weak gravity.